Allergies affect humans and companion animals and some allergic reactions (for example, those to insects, foods, latex, and drugs) can be so severe as to be life threatening.
Allergic reactions result when a subject's immune system responds to an allergen. Typically, there is no allergic reaction the first time a subject is exposed to a particular allergen. However, it is the initial response to an allergen that primes the system for subsequent allergic reactions. In particular, the allergen is taken up by antigen presenting cells (APCs; e.g., macrophages and dendritic cells) that degrade the allergen and then display allergen fragments to T-cells. T-cells, in particular CD4+ “helper” T-cells, respond by secreting a collection of cytokines that have effects on other immune system cells. The profile of cytokines secreted by responding CD4+ T-cells determines whether subsequent exposures to the allergen will induce allergic reactions. Two classes of CD4+ T-cells (Th1 and Th2; T-lymphocyte helper type) influence the type of immune response that is mounted against an allergen.
The Th1-type immune response involves the stimulation of cellular immunity to allergens and infectious agents and is characterized by the secretion of IL-2, IL-6, IL-12, IFN-gamma, and TNF-beta by CD4+ T helper cells and the production of IgG antibodies. Exposure of CD4+ T-cells to allergens can also activate the cells to develop into Th2 cells, which secrete IL-4, IL-5, IL-10, and IL-13. IL-4 production stimulates maturation of B cells that produce IgE antibodies specific for the allergen. These allergen-specific IgE antibodies attach mast cell and basophil receptors, where they initiate a rapid immune response to the next exposure to allergen. When the subject encounters the allergen a second time, the allergen is quickly bound by these surface-associated IgE molecules, resulting in the release of histamines and other substances that trigger allergic reactions. Subjects with high levels of IgE antibodies are known to be particularly prone to allergies.